1. Field of the Invention
Exemplary aspects of the present invention generally relate to a fixing device and an image forming apparatus including the same, and more particularly, to a fixing device that fixes a toner image on a recording medium, and an image forming apparatus, such as a copier, a facsimile machine, a printer, or a digital multi-functional system including a combination thereof, incorporating the fixing device.
2. Description of the Background Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member; an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may include a pair of looped belts or rollers, one being heated by a heater for melting toner (hereinafter referred to as “fixing member”) and the other being pressed against the fixing member (hereinafter referred to as “pressure member”). In a fixing process, the fixing member and the pressure member meet and press against each other, forming a so-called a fixing nip through which a recording medium is passed to fix a toner image thereon under heat and pressure.
During the fixing process, the melted toner in the toner image on the recording medium contacts the fixing member. In order to facilitate clean separation of the recording medium from the fixing member after the fixing process, the fixing member is often coated with fluorocarbon resin, and a separation pawl disposed downstream from the fixing nip is used to separate the recording medium physically from the fixing member.
Disadvantageously, however, the known separation pawl contacts the fixing member, and consequently, the separation pawl may damage the surface of the fixing member. When this happens, an output image has undesirable streaks appearing therein.
To address such a problem, a generally-known monochrome image forming apparatus employs a fixing member made of metal roller coated with Teflon (registered trademark). In this configuration, the fixing roller is prevented from getting easily damaged even when the separation pawl contacts the fixing member, thereby enhancing durability.
By contrast, in a case of a color image forming apparatus, the fixing member has a surface layer made of silicone rubber (generally, a PFA tube with a thickness of some tens of microns is used) coated with fluoride, or applied with oil to enhance color development. In this configuration, the surface layer is relatively soft and hence can be damaged easily by the separation pawl. Accordingly, color image forming apparatuses in recent years rarely employ such a separation pawl that directly contacts the fixing member to separate the recording medium therefrom, but instead employ a so-called contact-less separation method.
One such contact-less separation method includes injecting compressed air from nozzles against the end of the fixing nip to separate the leading end of the recording medium from the fixing member without damaging the surface of the fixing member.
Although advantageous, this approach has a drawback in that, because a recording medium bearing a solid image or a photo-image, or having a small margin tends to contain moisture, the recording medium sticks easily to the fixing member. Such a recording medium is difficult to separate from the fixing member unless the injected air acts directly on the recording medium.
In order to facilitate an understanding of the related art and of the novel features of the present invention, with reference to FIGS. 8 through 12, a description is provided of a related-art contactless separation method using an air separation mechanism. FIG. 8 is a schematic diagram illustrating a fixing device employing a related-art air separation mechanism. FIG. 9 is a schematic diagram illustrating separation of the recording medium using the related-art air separation mechanism illustrated in FIG. 8. FIG. 10 is a schematic diagram illustrating separation of the recording medium using the related-art air separation mechanism when the recording medium is conveyed while sticking to the fixing member. FIG. 11 is a plan view illustrating the related-art air separation mechanism. FIG. 12 is a graph showing a relation of a distance between an air supply source and a pressure at the tip of the nozzle (nozzle pressure).
As illustrated in FIG. 8, the related-art air separation mechanism includes an air supply tube 90 (shown in FIG. 11) and nozzles 87 that inject compressed air against a recording medium P discharging from the fixing nip between a pressure roller 81 and a fixing roller 82 to prevent the recording medium from sticking to the fixing roller 82 as the recording medium is discharged from the fixing nip as illustrated in FIG. 9.
Disadvantageously, however, the recording medium, which can be separated reliably from the fixing roller 82 as illustrated in FIG. 9, is limited to a relatively high basis-weight recording medium having a small amount of unfixed toner and hence less moisture. On the other hand, when discharging a relatively light basis-weight recording medium bearing a large amount of unfixed toner and moisture from the fixing nip, if the air is not injected against the leading end of the recording medium evenly in a longitudinal direction, the portion of the recording medium not exposed to air injection remains stuck to the fixing roller 82 as illustrated in FIG. 10. When this happens, the recording medium adhering to the fixing roller 82 keeps on being conveyed, and continues to apply heat to the recording medium unevenly. As a result, the unfixed toner on the recording medium is fixed unevenly in the longitudinal direction, causing an image defect.
In order to separate the recording medium from the fixing roller reliably, one conceivable solution may include increasing a number of nozzles along the longitudinal direction of the fixing roller so that the air is injected across the leading end of the recording medium in the longitudinal direction. However, in a configuration in which a plurality of nozzles, for example, at least 15 nozzles, are disposed on a air supply tube 90 as illustrated in FIG. 11, an amount of air pressure varies from nozzle to nozzle depending on the distance of the nozzle from the air supply source.
With reference to FIG. 12, a description is provided of the relation between distance from the air supply source to the nozzles and the air pressure of nozzles, based on a simulation performed by the present inventors. In FIG. 12, a double-headed arrow represents a recording medium passing area over which a recording medium passes.
In the air supply tube 90, a velocity of flow of compressed air is very fast and thus moves straight. Counterintuitively, however, the pressure of the compressed air near the nozzles near the air supply source drops because the compressed air moving in the air supply tube 90 draws the compressed air near the nozzles substantially near the air supply source, thereby reducing the pressure. As a result, the compressed air is not supplied sufficiently to the nozzles near the air supply source compared with the nozzles far from the air supply source. By contrast, the velocity of flow of the compressed air decreases near the nozzles far from the air supply source and a linearity of air flow is reduced. Accordingly, the compressed air is supplied to the nozzles smoothly.
In this configuration, the compressed air is not projected evenly from the plurality of nozzles, and hence the recording medium is not separated reliably from the fixing device.